This invention relates to a process for the purification of a gas stream at elevated pressure by nitrogen scrubbing. The gas stream and nitrogen are cooled to low temperatures and then the gas stream is scrubbed with liquid nitrogen. The purified gas stream is mixed with cold nitrogen which is under elevated pressure to form a mixture, in which nitrogen accumulates partly in liquid form. The thus formed gas mixture is reheated by heat exchange with the gas stream to be cooled and the nitrogen to be cooled.
Nitrogen scrubbing is used especially in the production of ammonia synthesis gas, in which a raw or impure hydrogen stream containing for example, small amounts of carbon monoxide, methane, argon and other components, which interfere with ammonia synthesis or otherwise represent undesirable materials, is scrubbed with liquid nitrogen. In this case, a purified hydrogen stream is produced which is laden with a part of the scrubbing nitrogen. Since the amount of nitrogen taken up by the hydrogen stream during scrubbing is not sufficient to achieve the stoichiometric ratio of nitrogen to hydrogen necessary for the ammonia synthesis, additional cold nitrogen is mixed with the product cold gas stream discharged from the scrubbing according to a known process and this mixture is then heated again (DE-PS No. 19 63 297, herein incorporated by reference).
In such a process, prior to the scrubbing step, the hydrogen stream and scrubbing nitrogen stream usually are cooled under basically the same pressure, for example at a pressure between 10 and 150 bars, especially at pressures between 20 and 80 bars. During cooling, the nitrogen is condensed or, the pressure level is above the critical pressure, is supercritically cooled, i.e. the density of the cold nitrogen increases substantially without phase transition. When it is fed into the scrubbing column or is mixed with the purified gas a mixture then results, which in any case is subcritical, as the critical point of this mixture of hydrogen and nitrogen is substantially higher than that of the pure components. As result a phase separation takes place and a liquid nitrogen-rich phase accumulates.
A disadvantage of the known process is that in the area of the cold end of the process, in other words in the coldest part of the heat exchanger upstream from the nitrogen scrubbing, large temperature differences occur. These large temperature differences are caused by the fact that the liquid nitrogen portion contained in the mixture is evaporated at its relatively low partial pressure and accordingly at a very low temperature, while the nitrogen to be cooled, which has a higher partial pressure is condensed at a higher temperature or supercritically cooled.
This is made clear by FIG. 1, in which the Q-T courses are qualitatively indicated. Q represents the heat gained or lost by the individual streams during heat exchange and T, of course, represent the temperatures of the streams. Thus, a sharp break occurs in the nitrogen cooling curve at the condensation temperature, which indeed is somewhat flattened at the supercritical state of the nitrogen, but without anything being qualitatively changed in this case, provided very high pressures are not used. The sharp break for cooling at a pressure below the critical pressure is given by the condensation enthalpy whereas the flattened shape of the curve for cooling at a pressure above the critical pressure reflects the increase of the specific heat content in the critical region.
On the other hand, evaporation of the nitrogen in the formed mixture takes place at variable temperature and consequently at a relatively flat course of the warming curve. As a result, large distances between the warming and cooling curve necessarily occur. The temperature differences thus occurring in the area of the cold end of the heat exchanger may reach values of over 20.degree. C. and therefore are connected with large energy losses.